My house, like the majority of houses built in the US over the last 50 or so years, has cheapo hollow core interior doors. (this particular house was on such a tight budget that even one of the exterior doors is hollow core). My son and I have been working room by room to upgrade the lighting, electrical and insulation, I guess the insulation cannot be considered an upgrade, since the house had none. We have also been ripping off and replacing the drywall in part to add insulation and to get rid of the horrible texturing job. After upgrading one of the bedrooms I wanted to change the on-suite bathroom and walk-in closet doors. The doors are 2'2" x 6'8" and we wanted to use single light door with obscure plastic. After being quoted $500.00 per door with a 2 month delivery date, I decided to make them myself. I recently acquired, for free, about 2,000 board feet of various hardwoods from a collector that had passed away. And out of that I selected three 2" x 14" x 7' planks of Philippine mahogany (also known as Luan). I happen to be making 2 identical doors.

Materials:
Wood for styles and rails (calculate this based on the size and number of doors).
(style refers to the vertical pieces, rails mean the horizontal pieces)
Dowels for the joints (in my case I used 8 1/2" x 3" per door)
Blue painters tape
2 part epoxy resin glue and microfiber thickener.

Warning this method of making doors uses sloppy fit dowels to compensate for a large degree of misalignment, based on my not having a horizontal drill press to properly align the holes. you must use a glue that gets its strength from a wide glue line, as far as I know epoxy is the only way to accomplish this. using any other kind of glue will result in failure.

Tools required:
Table Saw
Chop saw or radial arm saw
Sander (I prefer a DA but a belt sander in experienced hands will do)
Planer
Framing square
Drill press (big enough to fit you longest rail on end). I think you might be able to get away with using a hand drill with this sloppy fit system.
4 clamps (wide enough to clamp the width of the door)
mallet (to help align the door, when gluing up)
hammer and 1" chisel (to mortise the hinges)
Lock boring jig, also called a lock installation kit (Harbor Freight about $10.00)
Boat builders slick (optional, I use it because its one of my favorite tools)

Step 1: Cutting and Planing the Styles and Rails

I don't have a jointer so I straighten the boards on my table saw by nailing a straight edge to the fence side of the board and ripping the opposite side. once the boards are straight, I ripped them to 12" (save the offcuts and use them later for the stops) wide so they would fit in my planer. Once planed ( I made mine 1.5" thick) I then ripped the stiles to 6" and cut the rails bottom rail 12 inches and top rail 6" . the math is simple a 2' wide x 6'8" tall door opening for example would be 6" wide styles with 12 " wide rails. This will leave enough material to cut the door bevel and get you 1/16" to 1/8" clearance once the door is set in the jam.

Step 2: Dryfit and Mark the Dowel Locations

Lay your door out on a work table and mark the dowel locations 2 equal spaced dowels at each joint. be sure to use reference arrows and numbers to indicate the top of the door.

Step 3: Bore the Dowel Holes

For the sloppy fit method bore the holes 1/8" oversize. for example I'm using 1/2" dowels so I'm boring the holes 5/8". This will make up for any inaccuracy in your hole marks. You can always enlarge the hole to compensate more. bore the holes slightly deeper than the length of your dowels in my case I used 3 inch dowels so my holes were 1 5/8" deep

Step 4: Dryfit and Glue

Assemble the door on the table again, insert the dowels and clamp it together then use a straight edge to check alignment and a framing square to check for square. once your happy with the results, disassemble the door and mask the joints with blue painters tape.

Then mix your epoxy. mixing is critical so follow the directions, and then when you think you have mixed enough, mix some more. then add the microfibers thickener. add enough to make it the consistency of toothpaste.

Work quickly on the assembly shmoosh a lot of the epoxy paste into the holes and on the unmasked part of the joint (both side's). Put it all together and clamp top and bottom. double check for square. the clamps don't have to be to tight and you can use a mallet to do the final alignment .

Let dry overnight.

Step 5: Finishing Up

Remove as much tape as you can by hand then scrape and sand the rest of the tape and epoxy off. Lightly sand the whole door.

Rip plane and sand enough 1/2" square wood to make the stops front and back for 1/8" safety glass or plastic. nail one side of the stops in leaving a 3/16" reveal, lay the glass or plastic in and install the other stops..

Measure your jam and mortise the hinges. dry fit your door mark for any trimming to be done.

trim the door and cut a 3 to 5 degree bevel on the lock side of the door.

Dry fit again mark the center of your lockset (center of the existing strike). Install the lock and you should be done.

You deserve a beer or 2 at this point.